Induced Pluripotent Stem Cells – A New Perspective toward... : Journal of Dental Research and Review (2024)

Introduction

The ability of cells to differentiate into any kind of cell is referred to as the pluripotency of that cell and these cells are called induced pluripotent stem cells (IPSCs). These cells are derived from any source such as skin or alveolar bone or blood cells. They can divide indefinitely and are capable of generating all three derivatives of germ layers. They can be reprogrammed back to their embryonic-like pluripotent state. To prevent ethical controversies patients’ own cells that are genetically identical and histocompatible are used to practice personalized regenerative medicine.

In the field of dentistry, IPSCs are gained from stem cells derived from exfoliated deciduous teeth, buccal mucosa fibroblasts, dental pulp stem cells (DPSCs), apical papilla stem cells, and gingival and periodontal ligament (PDL) fibroblasts. Hard and soft tissues targeted for regeneration in dentistry in general and prosthodontics, in particular, include the alveolar bone, organs such as tongue and salivary glands, and structures such as condylar cartilage of temporomandibular joint (TMJ) and cranioskeletal muscles. In implantology, the regeneration of bone can specifically be used to improve bone formation around implants and to resolve the peri-implant bone defect.

The properties of pluripotency rely on two factors: potency and self-renewal. Under specific conditions, pluripotent stem cells can differentiate into specialized cells that are derived from all three germ layers ectoderm endoderm, and mesoderm.^[1] Hence, this review article was designed to select the relevant literature indicating the use, source, and generation of IPSCs, discussing the advantages and disadvantages derived from the conclusions drawn from various studies.

Materials and Methods

Selection criteria

An electronic search based on the National Center for Biotechnology Information (via PubMed), Medline, semantic scholar, BioMed Central, Directory of Open Access Journals, Science Open, and Bielefeld Academic was done up to the year 2020. The keywords for research were “stem cells in dentistry,” “IPSCs,” “embryonic stem cells,” “prosthodontic regeneration using induced pluripotent cells,” “stem cells from the PDL,” “dental pulp,” “deciduous tooth,” and “dental follicle.”

The research focus was on the introduction of IPSCs in dentistry, especially in prosthodontics, their source, and what was the need for the introduction of IPSCs.

Inclusion criteria

The complete data of this article were selected according to the following criteria:

1. Detailed information on IPSCs in dentistry and prosthodontics
2. How and why stem cells are programmed to IPSCs, and the related retrospective and prospective studies
3. Studies where IPSCs are used for the regeneration of various organs in dentistry and their future in implantology
4. Studies on advantages and disadvantages of IPSCs in dentistry
5. Only animal trials and descriptive, experimental, and cross-sectional studies were included in the review.

Exclusion criteria

1. Articles that are irrelevant for inclusion as IPSCs in dentistry
2. This study is limited to the head, neck, and face region of the body, it does not talk about the regeneration of any other body parts
3. This study does not hold any information on human trials.

Results

All articles that were relevant to the study were organized according to their subheadings in the article in order of their publication date. Sixty-nine articles were selected and included in the study. The results and conclusions were reviewed. No meta-analysis was done keeping in mind the narrative nature of the review.

Types of stem cells

Stem cells are of two types

1 Embryonic/fetal stem cells

2 Adult/somatic stem cells.

Embryonic stem cells (ESCs) are obtained from the inner cell mass of embryos.^[2,3] They have the potential to generate all three kinds of germ layers endoderm, ectoderm, and mesoderm. There is a restriction in the use of ESCs as they are gained from human embryos, so they are ethically and morally controversial and it is hard to generate disease and/or patient-specific ES cells.

Adult stem cells are somatic in nature and show the differentiation of the cells of their particular origin unless they are induced. Hematopoietic stem cells form blood-forming cells, which are found in the bone marrow and are responsible for the formation of blood cells and cannot form nerve cells or adipose cells. Similarly, adipose stem cells cannot form blood cells.

Therefore, IPSCs come into the picture, which functions in the same manner as ESCs. These IPSCs are the source of unlimited undifferentiated stem cells that can be differentiated into any kind of human cell as per the therapeutic requirement.

By direct reprogramming, pluripotency can be induced in somatic cells, which can be an alternative to ESCs.^[4] IPSCs were originally produced from mouse dermal fibroblasts (DFs) through retroviral gene transfer with the four reprogramming factors Sox2, Oct3/4, c-Myc, and Klf4. With the discovery of IPSCs, there is increased substantial demand for this technology as it does not require a human embryo for its pluripotency property and this property can be induced into any human stem cell.^[5]

While IPSCs may be obtained from a variety of adult tissues, dental tissues present an interesting source of precursors because of tissue accessibility and easy availability. Literature suggests that IPSCs can be differentiated back into their original cell type due to their genetic and epigenetic memory, so dental tissues when derived from IPSCs can differentiate back to their original dental tissues. Some studies suggest that IPSCs do maintain a memory of their original cell type,^[6] while other studies have shown that this memory is lost during in vitro culture.^[7] It is a dispute in the literature whether IPSCs do retain epigenetic memory owning to the original cell type.

Regeneration of lost oral tissues is the focus of IPSCs research in dentistry. Oral tissues contain a variety of stem cells, such as DPSCs, bone marrow mesenchymal stem cells (MSCs), shedding deciduous DPSCs, apical papilla stem cells, adipose stem cells, and PDL stem cells.^[8,9] A prosthodontist’s prime concern is the alveolar ridge height restoration because after the tooth loss there is a significant amount of bone defects which usually results in further vertical and horizontal bone loss,^[10] which in turn impedes the success of dental implants and other prosthodontic treatments.^[11] With the use of stem cells, defects and diseases of the craniomaxillofacial region such as hard and soft oral tissue defects, bone defects of the maxilla, and mandible and TMJ problems can be corrected. However, there is no such proven case of regeneration in a healthy individual, but animal tests have shown that regeneration is possible, and thus stem cells are the next-generation regenerative treatment, or in other words, stem cells are the future medicines.

Source of induced pluripotent stem cells in dentistry

When it involves stem cells, we are generally unaware of noninvasive ways of extracting them. IPSCs are often generated without cutting the living tissues. Dental stem cells can be easily obtained from the oral mucosa or gingival tissues and can be attained by wiping the oral mucosa with a cotton swab or could also be extracted from deciduous teeth or third molar which is considered biomedical waste. There are abundant stem cells in dental tissues such as PDL, dental pulp, and apical papilla and they can be differentiated into IPSCs.

In the oral cavity, stem cells are extracted from different areas of the teeth. They are as follows:

1 DPSC

2 Human exfoliated deciduous tooth

3 Stem cells from the apical papilla (SCAP)

4 Stem cells from the dental follicle

5 PDL stem cells (PDLSC)

6 Bone marrow-derived MSCs.^[12]

Silvia et al. presented a novel method of cryopreservation of DPSC in an intact tooth. Their method included cryopreservation without the purification of the cells, thereby reducing the start-up cost and workload of tooth banking. DPSCs were extracted from cryopreserved teeth through laser piercing. They showed MSC morphology, viable immune phenotype, and proliferation rate similar to the cells isolated from fresh, noncryopreserved teeth. Whereas cells isolated without laser piercing from the cryopreserved teeth showed significant loss of cell proliferation and viability.^[13]

How and why induced pluripotent stem cell?

In 2006, Takahashi and Yamanaka, using adult and embryonic fibroblasts from mice through ectopic co-expression of four genes, i.e., Sox2, Oct4, c-Myc, and Klf4, reported the successful extraction of IPSCs. It was enough for somatic cells to reprogram into an ESC-like pluripotent state through the expression of these four genes. The source material for IP cell line generations was taken from different species such as mice, rats, rhesus monkeys, and humans. Reprogramming was successfully translated to a tremendous number of other types of cells, which demonstrated a general capacity to alter their cellular identity. These cells were beta-cells of the pancreas, neural stem cells, melanocytes, mature B cells, cells from the stomach and liver, keratinocytes, and adipose stem cells.^[14] The customarily used DFs and factors such as KLF4 and c-Myc and/or ESC marker with its highest expression of endogenous reprogramming factors showed lesser reprogramming efficiency than DPSCs. As IPSCs generated from dental tissues are easily accessible by the dentist, dental tissues/cells held a bright source for the regeneration of tissues. It involved the induction of genes through vectors, normally retroviruses to actively convert adult cells into pluripotent cells. Utmost care should be taken to prevent viral integration into the host, inducing tumor formation.^[15]

Use of Induced pluripotent stem cell in dentistry and prosthodontics

The action of stem cells is their capacity to self-renew and differentiate into every organ and tissue. The target of IPSC research is to recover and rejuvenate all the tissues that have been lost. As we know, there is a sufficient amount of bone defects seen in the region of tooth loss, therefore limiting the successful treatment outcomes with dental implants and prosthodontic therapies. The main aim of any prosthodontist is the rehabilitation of alveolar bone height.

Regeneration of tooth/root

Stem cell technology has also gained popularity by replacing missing teeth with bioengineered teeth. In the field of prosthodontics, regeneration of the root is a more practical and clinically acceptable proposition, especially for fixed-prosthodontic treatments, like crown and bridge as the root generated can be used as a good abutment. Sonoyama et al.^[16] proved that using SCAP, PDLSCs, and hydroxyapatite and tricalcium phosphate scaffold, a root/periodontal complex could be constructed that can hold an artificial crown in a swine model that functioned as normal teeth. Various kinds of stem cells from mice, rats, and pigs have been used for tissue engineering to make dental structures in vivo. Ikeda et al.^[17] illustrated the transplantation of bioengineered tooth germ into the alveolar process of mice. The bioengineered tooth germ regenerated from epithelial and MSCs in a collagen gel resulting in a fully functioning tooth in mice. The bioengineered tooth formed showed proper tooth structure with the hardness of mineralized tissues for chewing, and it also responded to a noxious stimulus such as pain and mechanical stress. In this regard, IPSC has proven to be effective for tooth/root regeneration.

Salivary gland regeneration

For the treatment of salivary gland diseases, therapeutic and regenerative potentials of IPSCs were extrapolated. In vivo studies have been done in mice using IPSCs for the treatment of salivary gland carcinoma. With the use of IPSC technology, the activity (expression of a gene) of α-amylase has increased significantly, indicating improved salivary gland function. Even with increased activity, the gland showed malignant degenerative changes in its minor ductal, acinar, and vascular structures.^[18]

To restore damaged salivary glands functionally, two main regenerative approaches were formerly used. The first approach was by using tissue engineering technology, the artificial salivary gland could be developed, and the second approach was to use stem cells for the treatment of the mutilated salivary gland tissue. In a mouse model, submandibular gland functions were restored by transplanting and inducing adipose-derived MSCs in the irradiated submandibular gland.^[19] Successful reparation of the function of irradiated salivary glands by intraglandular transplantation of salivary gland stem cells isolated from the mice has been performed recently^[20] suggesting that the damaged salivary glands can be functionally repaired through transplantation of induced stem cells [Figure 1].

Alveolar bone/regeneration

The use of bone grafts has been the gold standard for the reconstruction of bone defects, but it also has side effects such as donor site morbidity, and bone resorption, and also the grafts may not be available in sufficient amounts. Using IPSC technology one can substitute autogenous grafting with bone tissue engineering. In this method, somatic cells of patients were induced to differentiate into bone-forming cells which were then loaded on a suitable scaffold in conjunction with proper bioactive molecules.^[21] A variety of mediums such as ascorbic acid, osteogenic media, bone morphogenetic proteins, ß-glycerophosphate, dexamethasone, and Vitamin D3 were proposed in isolation or in combination, to induce osteogenic differentiation of IPSCs. In an in vitro study with the assistance of OCT3/4, KLF4, c-MYC, and SOX2 transduction, fibroblasts taken from gingival tissue were reprogrammed to become stem cells. After reprogramming, these cells were then introduced into a titanium disc present in osteogenic media. Observation of osteogenic cells was seen after 28 days.^[22] The newly formed bone tissue had the same vascularization as that of the actually formed bone. A study was done using a calcium phosphate cement scaffold on male athymic rats. This scaffold contained three types of cells: pericytes, vascular endothelial cells, and IPSCs of which IPSCs have proven to be more successful in promoting bone regeneration There was greater bone regeneration improvement in animal bone-deficient models with the transplantation of stem cell populations within bioengineered three-dimensional scaffolds.^[23]

Temporomandibular joint regeneration

IPSC technology may be useful to study the complex etiology, onset, and progression of temporomandibular disorder disease on a cellular level.^[24] The current research has shown that reprogramming, synoviocytes and chondrocyte cells from osteoarthritis patients to IPSCs can lead to the formation of chondrocyte cells that deposit cartilaginous matrix.^[25] Cartilage can be regenerated from lineage-restricted MSCs and chondroprogenitors cells obtained from Human ESCs and IPSCs which holds a more promising approach that does not lead to teratoma formation. MSCs and chondroprogenitors cells obtained from human pluripotent stem cells, constitute a promising future to obtain “off-the-shelf” available cell sources in large numbers thereby overcoming the issues of donor variability and cellular senescence of adult MSCs.^[26]

Pluripotent stem cell and periodontal tissue regeneration [Figure 2]

Regeneration of the periodontal tissues using IPSCs was first done on mice by implanting the IPSCs into surgically created periodontal fenestration defects. This investigation comprises regeneration of periodontal tissues using IPSCs along with enamel matrix derivatives (EMD)/Emdogain gel.^[27] Emdogain gel exhibits greater regeneration of periodontal tissues by stimulation of multiple MSCs with enhanced expression of reprogramming tissue-specific factors. Periodontal defects treated with combined implantation of Emdogain gel with MiceIPSC (mIPSC) have shown greater bone regeneration than the defects treated with EMD_TM alone. Moreover, defects treated with EMD_TM and mIPSC showed almost an intact layer of regenerated cementum with a newly formed PDL between the regenerated cementum and alveolar bone, as was not shown on the control defects. This study, therefore, shows the capacity of mIPSC to regenerate PDL tissues.^[28]

Induced pluripotent stem cell and oral cancer

One of the most challenging fields is the treatment of oral cancer, which requires an average treatment outcome with minimal morbidity and death rate. Autologous immune cells such as anti-tumor monoclonal antibodies, natural killer (NK) cells, and adoptive transfer for the regression of solid tumors are used for the treatment of cancer.^[29] For cancer immunotherapy, NK cells play a major role as they are innate immune cells or lymphocytes which show an anticancer potential, whereas adoptive transfer requires in vitro isolation and expansion of T-cells and putting it back into the patients. Using IPSC for cancer immunotherapy is greatly achievable by the generation of large-scale NK cells.^[30] The research in the field of oncology, especially for oral cancer using IPSC will be perpetual to achieve better treatment modality and has a big advantage of its patient-specific nature.

The Future of implantology-induced pluripotent stem cell dental implants

Advancements in dental implants have come up rapidly in recent years to provide a proper dimension to functions, esthetics, and health issues due to missing teeth. The use of conventional dental implants takes a prolonged healing period with a rejection rate of 5%, which is not a perfect solution for replacing missing teeth. The use of IPSC in implant dentistry with its undifferentiated specificity could rather be the future of implant dentistry.

SongtaoShi^[31] conducted a study on swine for 6 months where he proved that stem cells when placed in the area of the removed tooth showed greater implant anchorage.

Stem cells for these implants are derived from the cells of the root apex contributing to better tissue regeneration and the ability to form bonds with the bone. These innate implants formed are called bio-roots. The bio-roots have an advantage over conventional implants in that they do not pose the risk of gum disease or loosening, which is generally a complication seen with conventional implants.^[32]

Research is going on to modify the technique of acquiring and using stem cells and make them cost-effective. Stem cell dental implant technology has just come up and is currently not a treatment option for replacing missing teeth. Yet the future lies in stem cell dental implants to provide a greater success rate and substantial longevity.

Advantages of induced pluripotent stem cell^[15]

1. Genetic and epigenetic memories can be retained using patient-specific IPSCs and this too reduces the probability of immune rejection along with host versus graft reaction
2. The use of IPSC avoids ethical issues, as they are obtained from the patient’s tissues
3. IPS cells are the same as ES cells in many ways, such as cell morphology, long telomeres, expression of pluripotency, and the ability to make embryoid bodies, viable chimeras (person with two genetically distinct types of cells derived from one zygote), and teratoma
4. Besides the use in cell-based therapy, IPS cells when derived from diseased patients become an efficient tool in understanding the mechanism of disease including cancer.

Disadvantages of induced pluripotent stem cell

1 The drawbacks in the use of IPSCs are mostly related to their current reprogramming methods. The integration of multiple viruses into IPS cell genomes by viral vectors has been employed for cell delivery, leading to tumorigenesis because of genetic abnormalities of the cells^[33]

2 Second, IPSCs, like ESCs, can lead to the formation of teratoma if they do not fully differentiate into somatic cells. Even though efficient differentiation protocols or techniques are developed, the separation of the differentiated cells from undifferentiated IPSCs is still a task with currently available cell purification technologies. This may increase the chance of transplanting undifferentiated or partially differentiated IPSCs into the patient^[34]

3 Genomic instability and aberrations

4 Increased risk of development of cancer.

Conclusion

With the advent of technology, the seed of IPSC is budding at a faster rate. IPSCs can generate almost all the kinds of cells required for the study and/or treatment of any kind of disorder. It can be used for the genetic correction of oral disease. It has promised its role in the regeneration of various organs such as the tongue, and TMJ. Nonetheless, their human trials are still going on and the animal trial results are quite positive.

When highly efficient and safe protocols for the regeneration of reprogrammed stem cells will develop then IPSCs are going to be the future of the treatment, therefore a perfect alternative.

Financial support and sponsorship

My mother Seema Devi Khandelwal is my financial support, and she is sponsoring the same.

Conflicts of interest

There are no conflicts of interest.

References